A “fuel cell” is a device which converts chemical energy into electrical energy. In a typical fuel cell, a gaseous fuel, such as hydrogen, is fed to an anode (the negative electrode), while an oxidant, such as oxygen, is fed to a cathode (the positive electrode). Oxidation of the fuel at the anode causes a release of electrons from the fuel into an electrically conducting external circuit which connects the anode and cathode. In turn, the oxidant is reduced at the cathode using the electrons provided by the oxidized fuel.
The electrical circuit is completed by the flow of ions through an electrolyte that allows chemical interaction between the electrodes. The electrolyte is typically in the form of a proton-conducting polymer membrane. The proton-conducting membrane separates the anode and cathode compartments while allowing the flow of protons between them. A well-known example of such a proton-conducting membrane is NAFION®.
A fuel cell, although having components and characteristics similar to those of a typical battery, differs in several respects. A battery is an energy storage device whose available energy is determined by the amount of chemical reactant stored within the battery itself. The battery will cease to produce electrical energy when the stored chemical reactants are consumed. In contrast, the fuel cell is an energy conversion device that theoretically has the capability of producing electrical energy for as long as the fuel and oxidant are supplied to the electrodes.
In a hydrogen/oxygen fuel cell, hydrogen is supplied to the anode and oxygen is supplied to the cathode. Hydrogen molecules are oxidized to form protons while releasing electrons into the external circuit. Oxygen molecules are reduced at the cathode to form reduced oxygen species. Protons travel across the proton-conducting membrane to the cathode compartment to react with reduced oxygen species, thereby forming water. The reactions in a typical hydrogen/oxygen fuel cell are as follows:Anode: 2H2→4H++4e−  (1)Cathode: O2+4H++4e−→2H2O  (2)Net Reaction: 2H2+O2→2H2O  (3)
In many fuel cell systems, a hydrogen fuel is produced by converting a hydrocarbon-based fuel such as methane, or an oxygenated hydrocarbon fuel such as methanol, to hydrogen in a process known as “reforming”. The reforming process typically involves the reaction of such fuels with water along with the application of heat. By this reaction, hydrogen is produced. The byproducts of carbon dioxide and carbon monoxide typically accompany the production of hydrogen in the reformation process.
Other fuel cells, known as “direct” or “non-reformed” fuel cells, directly oxidize fuels high in hydrogen content. For example, it has been known for some time that lower primary alcohols, particularly methanol, can be oxidized directly. Due to the advantage of bypassing the reformation step, much effort has gone into the development of so-called “direct methanol oxidation” fuel cells.
In order for the oxidation and reduction reactions in a fuel cell to occur at useful rates and at desired potentials, electrocatalysts are required. Electrocatalysts are catalysts that promote the rates of electrochemical reactions, and thus, allow fuel cells to operate at lower potentials. Accordingly, in the absence of an electrocatalyst, a typical electrode reaction would occur, if at all, only at very high potentials. Due to the high catalytic nature of platinum, platinum and its alloys are preferred as electrocatalysts in the anodes and cathodes of fuel cells.
However, a significant obstacle in commercializing fuel cells is the lack of stability of platinum electrocatalysts in the cathode during operation of the fuel cell. Typically, during operation of a fuel cell, the cathode potential will vary between approximately 0.5 and 1.1 V. This cathode potential variation is often caused by the fluctuating power requirements of a device powered by the fuel cell. For example, an automobile operated by a fuel cell requires stopping and starting.
During the higher cathode potentials, circa one volt, a portion of the platinum electrocatalyst has a tendency to oxidize, thereby causing the concomitant dissolution of platinum ions. The platinum ions are able to migrate at least as far as the proton conducting membrane. Hydrogen crossing through the proton conducting membrane from the anode causes the subsequent reduction of the platinum ions into platinum nanoparticles.
Accordingly, platinum is depleted from the cathode while the build up of platinum on the proton conducting membrane impedes the transport of hydrogen to the cathode. These effects cause a decline in the fuel cell's efficiency.
Another problem in existing electrocatalyst technology is the high platinum loading in fuel cell cathodes. Since platinum is a high-cost precious metal, high platinum loading translates to high costs of manufacture. Accordingly, there have been efforts to reduce the amount of platinum in electrocatalysts.
Platinum nanoparticles have been studied as electrocatalysts. See, for example, U.S. Pat. No. 6,007,934 to Auer et al.; and U.S. Pat. No. 4,031,292 to Hervert.
Platinum-palladium alloy nanoparticles have also been studied. See, for example, U.S. Pat. No. 6,232,264; Solla-Gullon, J., et al, “Electrochemical And Electrocatalytic Behaviour Of Platinum-Palladium Nanoparticle Alloys”, Electrochem. Commun., 4, 9: 716 (2002); and Holmberg, K., “Surfactant-Templated Nanomaterials Synthesis”, J. Colloid Interface Sci., 274: 355 (2004).
Other platinum-alloy compositions have been studied. For example, U.S. Pat. No. 5,759,944 to Buchanan et al. discloses platinum-nickel and platinum-nickel-gold electrocatalyst compositions.
None of the art discussed above discloses platinum or other noble metal electrocatalysts resistant to the oxidation and dissolution effects of fuel cells. Yet, there is a need for new electrocatalysts having such resistance, especially since such electrocatalysts would further the commercialization of fuel cells. The present invention relates to such electrocatalysts.